Packet distribution and selection in soft handoff for IP-based base stations among multiple subnets

ABSTRACT

A technique for assigning an address (“shadow address”) to a mobile station that is compatible with the layer-2 address on the wireline network which serves the mobile station. The shadow address is then used as a wireline identifier for the destination address for frames ultimately destined for the mobile station. The shadow address is stored in a watch list for serving base stations, and any base station receiving a frame with a shadow address in its watch list process the frame to forward it the to mobile station. In this way, the shadow address facilitates carrying out soft handoff and smooth handoff.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of provisionalapplication Ser. No. 60/281,308 filed Apr. 4, 2001. It is also relatedto Agrawal-Chen-Zhang application Ser. No. 09/975,801, filed Oct. 12,2001; Agrawal-Chen-Zhang application Ser. No. 09/975,890, filed Oct. 12,2001; Chen-Zhang application Ser. No. 09/975,910, filed Oct. 12, 2001;and Agrawal-Chen-Zhang application Ser. No. 09/976,166, filed Oct. 12,2001.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

This invention relates generally to a wireless-to-wireline andwireless-to-wireless communication system that is composed of wirelessaccess networks interconnected via a wireline IP (Internet Protocol)network, and, more particularly, to methodologies and concomitantcircuitry for effecting soft handoff in the wireless portion of thesystem.

2. Description of the Background Art

Today, many different wireless systems exist, ranging from indoorwireless LANs (Local Area Networks) to outdoor cellular systems.Generally, the numerous wireless systems are not compatible with eachother, making it difficult to roam from one system to another. Althoughthere have been attempts to unify third-generation wireless systems,incompatible systems are expected to co-exist in the future.Furthermore, wireless LANs and cellular wireless systems are beingdeveloped independently, and such systems are also evolvingindependently. So far, no wireless technology has emerged as a commonand long-term universal solution.

IP (Internet Protocol), which is already a universal network-layerprotocol for packet networks, is rapidly becoming a promising universalnetwork-layer protocol for wireless systems. An IP terminal, withmultiple radio interfaces, can roam between different wireless systemsif they all support IP as a common network layer. Unlike today'swireless systems in which Radio Access Networks (RANs) are mostlyproprietary, IP provides an open interface and promotes an open market.IP will also enable widely adopted and rapidly growing IP-basedapplications to run over wireless networks. Moreover, distributed,autonomous IP-based wireless base stations have the potential of makingthe wireless systems more robust, scalable, and cost effective.

There are, however, many challenges to realizing distributed all-IPwireless networks. For the sake of specificity in discussing thesechallenges as well as pointing out problem areas, reference is made toFIG. 1. The depiction of network 100 in FIG. 1 illustrates an exemplaryconfiguration of a network that uses IP-based wireless base stations(designated iBSs). The coverage area of the wireless network is definedby a multiplicity of cells (e.g., cells 101, 102, 103). The geographicalarea covered by each wireless base station is referred to as a cell(e.g., iBS 111 serves cell 101, and so forth). When mobile station 104moves from one cell (e.g., cell 101 originally) into the overlappingregions (e.g., overlap of cells 101 and 102) of the coverage areas ofmultiple base stations, base station 111 may perform a “handoff” ofmobile station 104 to base station 112. Handoff is a process whereby amobile station communicating with one wireless base station is switchedto another base station during a session. Overlap regions 117 and 118are coverage areas where handoff is effected. For example, as mobilestation 104 moves into region 117 while roaming in cell 101, the radiosignal strength from iBS 2 (depicted by reference numeral 115) may begreater than the radio signal strength (114) from iBS 1, so handoff iswarranted to maintain the quality of the established session.

Among the key challenges in a distributed all-IP wireless network is howto support “soft handoff”. As suggested above, handoff is the processthat allows a mobile station's session-in-progress to continue withoutinterruption when a mobile station (MS) moves from one wireless cell toanother. Soft handoff is a form of handoff whereby a mobile station canstart communication with the target base stations without interruptingthe communication with the serving base station. Thus, soft handoffallows a MS to communicate with multiple base stations (BSs)simultaneously. In particular, soft handoff has been shown to be aneffective way for increasing the capacity, reliability, and coveragerange of CDMA-based wireless networks. Soft handoff also provides moretime for carrying out the handoff procedure.

Soft handoff in a CDMA-based wireless system is the focus of the subjectmatter of the present invention. In Code Division Multiple Access (CDMA)radio systems, a narrowband user message signal is multiplied by a verylarge bandwidth signal called the spreading signal. The spreading signalis a pseudo-noise code sequence that has a communication signal ratewhich is orders of magnitudes greater than the data rate of the usermessage signal. All users in a CDMA system may transmit simultaneously.Each user has its own pseudorandom code for coding its own messagesignal—each code is approximately orthogonal to all other codes. Areceiver is assigned a code to detect a desired user message signal, andperforms a time correlation operation to detect only the specificassigned code. All other codes appear as noise due to de-correlation.CDMA is effective in wireless systems because a receiver can be assigneda multiplicity of codes to detect message signals from a correspondingmultiplicity of transmitters, thereby engendering the soft handoffprocess.

An IP router is an IP network device that runs IP layer routing protocol(e.g., OSPF and BGP) and forwards IP packets. The running of a routingprotocol decides the “routing policy”, and the forwarding of IP packetsrealizes the “routing mechanism”. IP packets arriving from the wirelineIP network (121) at a given base station (e.g., iBS 111 over wirelinepath 122 or iBS 112 over path 123) can be routed by the routingmechanism of the base station to mobile station 104 (or otherappropriate wireline devices that connect directly to the base station).

Today, the only known approach to designing an IP-based base station isto add (or connect) radio transmission and receiving equipment directlyonto an IP router (131). Such a design, however, has a potentiallyserious shortcoming. In particular, the mobile stations served bydifferent base stations must belong to different IP subnets, that is,the design forces the mobile stations in different cells to be ondifferent IP subnets. (Here, a subnet is used in the sense defined by anIP address, which has the form, for example, “w.x.y.z” (e.g.,129.3.2.14), wherein “w.x” is the network address (129.3), “y” (2) isthe subnet address for a device associated with the given network, and“z” (14) is the host address for a device associated with the givennetwork/subnet, such as a mobile terminal or a base station. In terms ofFIG. 1, iBS 1 may be assigned the subnet address 2, whereas mobilestation 104 may have the host address 14.) Suppose, for the sake ofargument, that a mobile station is served by two base stations belongingto the same IP subnet S. Then, both iBSs (IP routers) will advertise toother routers in the overall network that they can reach all the hostson subnet S. However, each iBS can only reach a subset of the hosts onsubnet S (i.e., the set of hosts being currently served by the basestation). This means that other routers will not be able to determinewhich base station should receive a packet destined for a host on subnetS. In other words, packets may be delivered to the wrong base stationand consequently cannot reach the destined host.

The fact that mobile stations (MSs) in different cells belong todifferent IP subnets suggests that an MS may have to change its IPaddress every time it moves into a new cell. Changing IP address usuallytakes a long time using today's methods for dynamic IP addressassignment (e.g., the Dynamic Host Configuration Protocol or DHCP). Whencertain IP-layer mobility management mechanisms are used (e.g.,SIP-based mobility management), a change of IP address can also meanthat the old session may need to be modified, or new SIP sessions mayhave to be established.

Having to change IP addresses when moving from one cell to another alsomakes soft handoff more difficult to implement. For example, if an MShas to use different IP addresses to receive IP packets from differentiBSs, IP packets coming to the MS from different iBSs will not beidentical because they carry different IP destination addresses.Consequently, copies of the same packets from different base stationsmay not be correctly combined by the MS's radio system.

Recently, methods (e.g., HAWAII, Cellular IP) have been proposed toenable MSs to move within a domain of multiple IP subnets without havingto change their IP addresses. These methods, however, typically requirecomplex IP-layer signaling and significant changes to the IP routers inthe domain. Furthermore, these methods have not considered how to solvethe data content synchronization problem.

From another viewpoint, in today's circuit-switched CDMA networks suchas IS-95, a centralized Selection and Distribution Unit (SDU) isresponsible for data distribution in the forward direction (from BS toMS). The SDU creates and distributes multiple streams of the same dataover layer-2 circuits to multiple BSs that in turn relay the data to theMS. The MS's radio system (typically working below the IP layer)collaborates with the BSs to synchronize the radio channel frames andcombine the radio signals received from different BSs to generate asingle final copy of received data. The SDU helps ensure data contentsynchronization by ensuring that the matching layer-2 frames sent todifferent base stations contain copies of the same data. In the reversedirection (from MS to BS), the MS ensures that the matching layer-2frames sent to different BSs contain copies of the same data. The SDUthen selects one of the frames received from different base stations asthe final copy of the data.

Accordingly, as evidenced by the foregoing discussion, achieving softhandoff among distributed iBSs introduces several new technical problemsthat cannot be solved readily by the mechanisms developed for today'scentralized circuit-switched wireless networks.

One problem already alluded to is loss of data content synchronization.With distributed iBSs, centralized control entities, such as the SDU incircuit switched wireless networks, will no longer exist. Consequently,even though the CDMA radio system is capable of synchronizing the linkand physical layer frames on the radio channel, it cannot, on its own,guarantee that the matching frames from different base stations willcarry copies of the same data. For example, IP packets can be lost ontheir way to the MS, creating random gaps in the packet streams receivedby the MS from different iBSs. Furthermore, copies of the same data mayarrive at the MS at different times due to the random delays suffered bythe packets. Random gaps and delays can lead to a loss of data contentsynchronization. Suppose that packet X is lost at iBS 1 (due to, forexample, buffer overflow) but is not lost at iBS 2. Then, anothertotally unrelated packet Y from iBS 1 and packet X from iBS 2 may arriveat the MS at the same time and the MS's radio system will not be able totell that they are not copies of the same data and will henceerroneously combine X with Y.

Another problem is how to support soft handoff, which requires a mobilestation to receive identical copies of the same data from multiple basestations simultaneously. When the mobile stations served by differentbase stations belong to different IP subnets, complex IP-layer signalingcapabilities (e.g., IP multicast) have conventionally be required todirect copies of the same IP packets via multiple base stations to themobile station. Furthermore, copies of the same IP packet arriving fromdifferent base stations to the mobile station will not be identicalbecause these packets will carry different destination IP addresses.This makes it impossible for the mobile station's radio system tocombine the signals from different base stations into a single copy ofdata.

The art is devoid of a methodology and concomitant systems that effectsoft handoff in an all-IP wireless network that uses autonomous iBSs ina configuration having the following characteristics that differentiatethe configuration from existing wireless networks: (a) the iBSs use IPprotocols for both signaling and transport of user traffic. For example,they may route/forward IP packets based on information carried in the IPheaders, perform IP-layer signaling, mobility management and Quality ofService (QoS) management functions; (b) the iBSs function autonomously.There is no centralized signaling and control over the behaviors of theiBSs; (c) the iBSs are interconnected via an IP network which could havearbitrary network topology such as bus, ring, star, tree, etc.; and (d)the cells (a cell is a geographical radio coverage area of a BS) can bearranged in any arbitrary configuration.

SUMMARY OF THE INVENTION

These shortcomings and other limitations and deficiencies are obviated,in accordance with the present invention, by assigning an alias orshadow address to a mobile station that is compatible with the linklayer address of the wireline subnet which delivers packets to themobile station via base stations connected to the wireline subnet,storing the shadow address in the base stations that serve the mobilestation during soft handoff, and using the shadow address of the mobilestation for packets communicated to the mobile station via the basestations from a sending device coupled to the subnet.

Broadly, in accordance with a method aspect of the present invention, amethod for carrying out soft handoff of a mobile station from a firstbase station served by a first wireline subnet to a second base stationserved by a second wireline subnet, wherein each subnet has a link layerdifferent than the link layer of the wireless network serving the mobilestation and the mobile station has initially a first IP addresscompatible with the first wireline subnet and wherein the first basestation is served by a nearest router, includes: (a) storing of thefirst IP address and a shadow address in the nearest router, the shadowaddress corresponding to the mobile station and having a formatcompatible with the link layer of the first wireline subnet; (b)assigning a second IP address to the mobile station compatible with thesecond wireline subnet; (c) communicating the second IP address from themobile station to nearest router via the first base station; and (d)transmitting a frame containing the packet as conveyed by the sendingdevice from the nearest router over the first wireline subnet to thefirst base station as determined by the shadow address and over thesecond wireline subnet to the second base station based upon the secondIP address.

Broadly, in accordance with a system aspect of the present invention, asystem for carrying out soft handoff of a mobile station from a firstbase station served by a first wireline subnet to a second base stationserved by a second wireline subnet, wherein each subnet has a link layerdifferent than the link layer of the wireless network serving the mobilestation and the mobile station has initially a first IP addresscompatible with the first wireline subnet, and wherein the first basestation is served by a nearest router, includes: (a) a storage devicefor storing the IP address and a shadow address of the first basestation in the nearest router, the shadow address corresponding to themobile station and having a format compatible with the link layer of thefirst wireline subnet; (b) a processor for assigning a second IP addressto the mobile station compatible with the second wireline subnet; (c) asending device for communicating the second IP address from the mobilestation to nearest router via the first base station; and (d) atransmitter for transmitting a frame containing the packet as conveyedby the sending device from the nearest router over the first wirelinesubnet to the first base station as determined by the shadow address andover the second wireline subnet to the second base station based uponthe second IP address.

BRIEF DESCRIPTION OF THE DRAWING

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a wireline-wireless system composed of autonomous basestations serving mobile stations;

FIG. 2 depicts a system composed of both wireline and wireless networkswherein base stations on a subnet serve mobile stations using theirshadow addresses;

FIGS. 3A and 3B depict a generic IP packet and an IP packet having amobile station as a destination device, respectively;

FIGS. 3C and 3D depict a generic Ethernet frame and an Ethernet framehaving the shadow address of the mobile station as an Ethernetdestination address, respectively;

FIG. 4 depicts a system composed of both wireline and wireless networkswherein a multiplicity of base stations on the same subnet send multiplecopies of a packet to a roaming mobile station to carry out softhandoff;

FIG. 5 depicts base station processing in terms of conventional physicallayer, link layer, and IP layer protocol stacks while using shadowaddresses of mobile stations;

FIG. 6 depicts the arrangement of a base station in terms of only theconventional physical layer and link layer protocol stacks to processshadow addresses of mobile stations;

FIG. 7 depicts the processing by a base station that performs therouting mechanism without changing the routing policy fostered by shadowaddresses;

FIG. 8 is a flow diagram depicting the process of assigning a shadowaddress to a mobile station;

FIG. 9 is a flow diagram depicting the process by which a mobile stationassociates with a new base station using physical layer information andlink layer messages;

FIG. 10 is a flow diagram depicting the process of assigning a shadowaddress by a base station upon first power-up of a mobile station;

FIG. 11 is a flow diagram depicting the process of assigning a shadowaddress by a candidate base station for a powered-up mobile stationhandled by a serving base station;

FIG. 12 is a flow diagram depicting the process of deleting a shadowaddress from a watch list;

FIG. 13 is a pictorial representation of a mobile station roaming fromone cellular region served by one subnet to another cellular regionserved by another subnet during the process of soft handoff;

FIG. 14 is a flow diagram depicting an illustrative technique forgenerating entries for the Packet Duplication table, including amultiplicity of IP layer address associated with a roaming mobilestation

FIG. 15 is a time diagram depicting the time relationship of packets andframes as delivered by two base stations to a mobile receiver involvedin soft handoff across subnets;

FIG. 16 is a pictorial representation illustrating recovery ofsynchronization for matchable packet streams during soft handoff; and

FIG. 17 is a flow diagram depicting the algorithm to regainsynchronization for matchable packet streams during soft handoff.

DETAILED DESCRIPTION

1. Soft Handoff Within a Subnet

1.1. “Shadow” Address

As illustrated by system 200 in FIG. 2, which is a recast version ofsystem 100 of FIG. 1 for purposes of highlighting the principles of thepresent invention, on one side of iBS 1 and iBS 2 (111 and 112,respectively) is wireline IP network 201. Wireline IP network 201 servesas one subnet used to interconnect iBS 1 and iBS 2 with router 131. Forsake of specificity, but without loss of generality, wireline IP network201 is presumed to be an Ethernet. Router 131 is also coupled to otherIP networks, such as the Internet, via another port to deliver packetsto and from wireline IP network 201 over path 132. On the other side ofiBS 1 and iBS 2 is a wireless network represented, in part, by wirelessradio path 114 linking MS 104 with iBS 1. Thus, there is a cleardemarcation between the wireline and wireless aspects of system 200,which is depicted in FIG. 2 by the Ethernet and non-Ethernet portions ofsystem 200.

It is also readily appreciated that, whereas the sending device wasexemplified as being coupled, for example, into wireline network 201from a device propagating a packet through router 131 of FIG. 2, it isclear that the principles of the present invention also apply for awireless sending device that use a link layer addressing scheme which iscompatible with the link layer of the subnet. Accordingly, both casesare covered in the description by stating that the sending device is“coupled to” the wireline subnet, thereby covering (but not beinglimited to) both direct connection or wireless coupling to the subnet.

To understand the importance of the separation between the wireline andwireless sides of the base stations, consider the ramifications of asingle IP subnet serving multiple iBSs, as illustrated in FIG. 2.Generically, when a device (e.g., router, iBS, or host) on the wirelineIP subnet wants to send an IP packet to a mobile station, the sendingdevice, by convention, normally determines the layer-2 address thatshould be used to send the IP packet over the wireline subnet to thebase station handling the mobile station. But, when the mobile stationis using a different layer-2 protocol that is incompatible with thelayer-2 protocol used on the wireline subnet, the sending device cannotuse the layer-2 address of the mobile to send IP packets over thewireline subnet. This will be the case, for example, when the wirelessnetwork uses CDMA technologies that have a different format for thelayer-2 address than that on the wireline subnet (such as Ethernet).

To circumvent this difficulty and to ensure that a sending device candirect the packet to the right base station, consider first deployingthe wireline layer-2 address (e.g., MAC address in the case of Ethernet)of the destination base station as an alias for the mobile station and,accordingly, the sending device would forward the packet to thedestination base station as the proxy for the mobile station. Then, thebase station can use, in turn, IP-layer and/or other layer informationto determine to which mobile station the packet should be sent. This isonly part of the solution, however, because in this scenario only asingle base station serves the mobile station. To carry out softhandoff, it is necessary that multiple base stations relay copies of thesame packet to the mobile station which would be difficult in the abovescenario, that is, sending a packet to only a single base station'swireline layer 2 address.

Consider now an extension to the above approach whereby a so-called“shadow address” is utilized. With the shadow address approach, besidesthe unique wireless layer-2 address normally allocated to each mobilestation, a unique wireline layer-2 address is also assigned to themobile station. Since the mobile station may be using a different layer2 than the wireline network, the wireline layer-2 address assigned tothe mobile station may have no meaning to the mobile station and cannotbe used by the mobile station for any other purposes. However, thewireline layer-2 address assigned to the mobile station can beadvantageously used by a base station to determine which layer-2 framearriving from the wireline network should be accepted by the basestation and, if accepted, to pass the IP packet contained in the layer-2frame to the IP layer in the base station for further processing. The IPlayer of the base station then uses the information in the IP header ofthe incoming IP packet to determine which radio interface on the basestation the packet should be sent to. In essence, the wireline layer-2address assigned to a mobile station can be viewed as a “shadow” thatthe mobile station casts on the wireline layer 2.For this reason, thewireline layer-2 address assigned to a mobile station will be referredto as the shadow wireline layer-2 address of the mobile station, or“shadow address” for short.

Each base station maintains a “watch list” similar to that in Table 1.The field denoted “MS's IP Address” contains the IP address which isassigned to the mobile station in the subnet. The field denoted “MS'sShadow Address” is the shadow wireline layer-2 address. Table 1 showsthe address in the format of the IEEE 802.3 MAC address which is themost popular link layer or layer 2 for a LAN (layer 2 and link layer areused interchangeably in the sequel without loss of generality). Thefield denoted “MS's Link Layer Address” is the real wireless link layeraddress of the mobile station. It could be the link layer address forany one of many different standards, such as cdma2000, WCDMA, Bluetooth,and so forth.

TABLE 1 MS's IP Address MS's Shadow Address MS's Link Layer Address128.33.22.121 00:60:1D:03:E7:E1 xxxxxx 129.55.32.131 E1:E7:03:1D:60:01yyyyyy . . . . . . . . . . . . . . . . . .

The shadow address may be assigned to a mobile station dynamically whenthe mobile station powers up and accesses the wireless network for thefirst time. Or, it can be configured in the mobile station prior to thefirst time it is powered, that is, each mobile station may be assigned ashadow address in addition to a real link layer address. Shadow addressassignment will be discussed in more detail below when flow diagrams arepresented.

1.2. Use of Shadow Address by a Base Station to Relay a Packet

Each base station uses the shadow address created for each mobilestation to help relay an IP packet between the wireless and the wirelinenetworks. The wireline interface of each base station examines thelayer-2 destination address of each layer-2 frame arriving from thewireline network. If the destination layer-2 address matches any shadowaddress in its watch list, the base station accepts the frame, takes theIP packet out from the frame, and passes the IP packet to the basestation's IP layer for further processing. For ease of discussion, theterm “matching frames” is used to refer to a layer-2 frame whosedestination layer-2 address matches one of the shadow addressescurrently in the base station's watch list. If the IP packet is destinedfor one of the mobile stations currently served by the base station, theIP layer forwards the IP packet to the destination mobile station. Ifthe IP packet is not destined for either any mobile station currentlybeing served by the base station or the base station itself, the IPlayer may ignore the packet.

When any device on the local wireline IP subnet wants to send a first IPpacket to a mobile station, the sending device must first determine theshadow wireline layer-2 address of the destination mobile station. Thesending device can do this, for example, by using the well-known,conventional Address Resolution Protocol (ARP) designed for the wirelineIP network. In particular, the sending device will broadcast an ARPREQUEST packet over the local IP subnet. The base station that has theshadow layer-2 address of the destination mobile station in its watchlist will respond to the ARP REQUEST with the shadow layer-2 address ofthe destination mobile station. The shadow wireline layer-2 address willthen be used by the sending device on the local IP wireline network tosend a packet to the mobile station via the base station responding tothe ARP REQUEST. In addition, when the shadow layer-2 address of adestination mobile station is in the watch list of multiple basestations, all of the base stations may respond to the ARP REQUEST. Theseresponses will contain the same shadow address of the destination mobilestation. The import of multiple base stations having the mobile stationon their watch lists will be elaborated upon shortly.

To illustrate the foregoing discussion concretely, consider the IPpacket of FIG. 3A composed generally an IP payload 303 and an IP headerwhich is composed of, among other information, the IP destinationaddress 301 and IP source address 302. In addition, suppose the packetis originated by an IP sending device coupled to path 132 of FIG. 2, andthe packet is destined for mobile station 104 served by iBS 1. Thus, theIP destination address is the IP address of the mobile station 104—thisparticular packet is shown in FIG. 3B. In order for the sending deviceto decide how to encapsulate the packet at layer 2, the sending devicepropagates an ARP REQUEST asking for the layer-2 address thatcorresponds to the IP address of the mobile station (e.g. 128.33.22.121in Table 1) over wireline IP network 201, which again for concreteness,is presumed to be an Ethernet. The ARP REQUEST reaches iBS 1, and iBS 1notes that this IP address is in its watch list and it is associatedwith mobile station 104. In response to the ARP REQUEST, iBS 1 sends theshadow address (00:60:1D:03:E7:E1) of mobile station 104 to the sendingdevice. The shadow address, which is an actual wireline layer-2 addressfor wireline 201, can then be used by the sending device to encapsulatethe packet into an Ethernet frame. A generic Ethernet frame is shown inFIG. 3C and has, besides the packet fields, the Ethernet destinationaddress 311 and the Ethernet source address 312. In FIG. 3D, the actualEthernet frame containing the given packet is shown; the Ethernetdestination address is the shadow address of mobile station 104.

As the Ethernet frame of FIG. 3D propagates over wireline 201, the frameis detected by iBS 1. Then iBS 1, via its watch list, recognizes theshadow address in the Ethernet frame as one being served by iBS 1.Accordingly, iBS 1 handles the Ethernet frame by stripping off thelayer-2 information, including fields 311 and 312, and passes the IPpacket to the IP layer processing of iBS 1. Processing at the IP layerwill be discussed in more detail in the sequel.

The layer-2 frames going out from a base station to the wireline IPnetwork may set its source layer-2 address to the shadow layer-2 addressof the source mobile station (the mobile station that originated thispacket) or the layer-2 address of the base station.

1.3. Base Stations Use of Shadow Addresses to Simultaneously RelayCopies of the Same Packet to a Mobile in Soft Handoff State

Arrangement 400 of FIG. 4 illustrates how multiple base stations can usethe shadow layer-2 address of a mobile station to simultaneously relaycopies of the same IP packet to the mobile station to carry out softhandoff.

Suppose initially that mobile station 104 is registered with iBS 1(111), and that mobile station (MS) 104 has the parameters listed in row1 of Table 1 above, that is, the IP address of the MS 104 is128.33.22.121 and the shadow address is 00:60:1D:03:E7:E1 (which will becalled MAC₁₀₄ for short). Watch list 1 (401) in iBS 1 is, forillustrative purposes, that exemplified by Table 1; accordingly, theshadow address of MS 104 is in watch list 401. When mobile station 104first starts communication with new base station iBS 2 (112) as it roamsinto the overlap of the cellular regions, iBS 2 will insert MAC₁₀₄ intoits watch list, shown as watch list 2 (402). Many ways exist for the newbase station to learn the shadow address of the MS and will be discussedin greater detail later. Exemplary contents for watch list 402 arelisted in Table 2 below; the third row contains information about MS104. From this point in time, iBS 2 will accept a layer-2 frame comingfrom the wireline network that carries MAC₁₀₄ as the destination layer-2address and will send the packet carried in this frame to the IP layeron iBS 2 for further processing.

TABLE 2 MS's IP Address MS's Shadow Address MS's Link Layer Address . .. . . . . . . 128.44.12.111 F1:F7:04:2D:70:03 zzzzzzzzz 128.33.22.12100:60:1D:03:E7:E1 xxxxxx . . . . . . . . .

Since both base stations iBS 1 and iBS 2 now have MAC₁₀₄ in their watchlists, they will both accept layer-2 frames destined for MAC₁₀₄ andforward the IP packet carried in these frames to the mobile stationsimultaneously, as exemplified by path 405 and path 406, respectively.Path 405 delivers Ethernet frame 404 containing MAC₁₀₄ and an embeddedIP packet over IP subnet 201 to iBS 1 and, in turn, over a radio channelto MS 104. Similarly, path 406 delivers IP frame 404 containing MAC₁₀₄and the embedded IP packet over IP subnet 201 to iBS 2 and, in turn,over a radio channel to MS 104.

FIG. 5 illustrates the arrangement 500 of a base station in terms ofconventional protocol stacks, namely, physical layers 501-1 and 501-2,link or layer 2 layers 502-1 and 502-2 (e.g., receiving frames), and theIP layers 503 (e.g., receiving packets) to process the shadow wirelinelayer-2 address for a mobile station. Layers 501-1 and 502-1 areassociated with the wireline side of the base station, whereas layers501-2 and 502-2 are associated with the wireless side of the basestation. By way of reiteration, the main purpose of shadow addresses isto enable the wireline interface of the base station to accept thelayer-2 frames that arrive from wireline networks and are destined formobile stations served currently by the base station with reference toits watch list. The wireline interface of the base station will monitorall layer-2 frames that come from the wireline network and will acceptany layer-2 frame whose destination layer-2 address matches the shadowaddress of any mobile station currently being served by the basestation. As depicted in FIG. 5, there are two incoming frames labeled511 and 521, respectively. It is presumed that only frame 512 has alayer-2 destination address that is in the watch list (505) for the basestation. In effect, the watch list acts as a filter to select only thoseframes having a destination address which is either the base stationlayer 2 address or shadow addresses contained in the watch list. Once alayer-2 frame is accepted by the wireline layer 2, the IP packet (522-1)carried in the frame will be extracted and passed to the IP layerforwarding engine 506 in IP layer 503 for any processing (e.g., QoScontrol) at the IP layer.

In particular, the IP address of the mobile station is known to the basestation via, for instance, the contents of watch list exemplified byTable 1. Moreover, the base station utilizes another table that mapsradio channels to mobile stations; an illustrative table is shown byTable 3:

TABLE 3 Radio Channel Number MS's IP Address MS's Link Layer Address 1not assigned . . . 2 128.33.22.121 xxxxxx . . . . . . . . . N129.55.32.131 yyyyyy

The IP address of mobile station 104, as used throughout the discussion,is on the second row of Table 3, namely, 128.33.22.121. Radio channel 2is presently serving mobile station 104. Outgoing packet 522-2 from theIP forwarding engine, which is the counterpart to incoming packet 522-1resulting from processing in IP forwarding engine 506, is passed tolayer 2 (502-2) for encapsulation. The frame format is that deployed bythe radio system, and the destination address is determined from themobile station's layer-2 address from watch list 505. Finally, the layer2 radio frame is propagated to mobile station 104 over wireless channel114.

A base station could also directly use the shadow addresses to determineto which mobile station a layer-2 frame arriving from the wirelinenetwork should be sent to and then send the layer-2 frame directly tothe outgoing radio channel without IP-layer processing. This process isreferred to as layer-2 switching, which means switching layer-2 framesfrom an incoming layer 2 to an outgoing layer 2 of a base station. Inthis mode, the base station is essentially functioning as a layer-2bridge.

The arrangement 600 of FIG. 6 depicts this scenario. As in FIG. 5, anincoming frame is passed to layer 2 (501-2) whenever the layer-2 addressof the incoming frame is in the watch list. Because the watch list hasthe necessary information to complete packet forwarding at layer 2,namely, the layer-2 wireline address (e.g., xxxxxx), the IP layerprocessing can be bypassed if desired. Again, the contents of Table 3can be use to identify the mobile station by its layer 2 address (606),and encapsulate the frame propagated by radio channel 531 using thelayer-2 wireless address (it is clear that a simplified version of Table3 is possible in this case, wherein only the first and third columnscompose the simplified table). Note that the base station switches thepayload of the incoming frame (e.g., IP packets in case of IP-based basestation) rather than the entire incoming layer-2 frame. This is becausewireless and wireline layer-2 protocols used in the network can becompletely different and, consequently, the layer-2 header on thewireline network will become useless in the wireless network and viceversa.

1.4. IP-Based Base Station Performs Routing Mechanism without ChangingRouting Policy

One operational principle in accordance with the present invention isthat each IP-based base station will act as an IP-layer forwarder, asalluded to in FIG. 5. Now referring to FIG. 7, there is shown apictorial representation of the processing effected by a base station sothat the routing mechanism based on shadow addresses does not require achange in the routing policy of a base station. In particular, in oneoperational mode, iBS 701 (representative of, say iBS 1 (111)) uses theinformation in the IP header of an incoming packet (e.g., IP 0) fromwireline interface 722 and a routing table to determine where the packetshould be sent, and then forwards the packets to the correct outgoingradio channel, i.e. iBS 701 station performs the IP routing mechanism.However, it is not required that a base station run IP routing protocolsto change its routing table. For instance, IP forwarding engine 707 maydetermine that IP 0 is bound for MS 1 (711) and, accordingly, forwardsIP 0 as outbound packet IP 1 to MS 1 via electronic/radio path 715 inradio interface 721 and radio path 713. A description similar to theabove also applies to MS 2 (712).

1.5. Assignment and Processing of Shadow Addresses

There exist many ways for a new base station to learn about or assignthe shadow address for a MS. For example, the iBSs may obtain the shadowaddress for a MS from a network server responsible for assigning shadowaddresses. Alternatively, and the focus in accordance with the presentinvention, is the case wherein the iBSs themselves can be responsiblefor assigning shadow addresses to MSs. In this case, the MS may carryits own shadow address and pass it along to the new base station or thenew base station may obtain a MS's shadow address from the serving basestation.

By way of elucidating the details of a representative technique forassigning and using a shadow address which illustrates the methodologywhereby the MS carries its shadow address, it is assumed in thefollowing description that: the serving iBS assigns a shadow address toa MS; the MS passes its assigned shadow address to a new iBS; and when aMS's shadow address conflicts with any shadow address currently in usein the new cell, the new iBS will negotiate with the serving iBS toresolve the conflict.

An illustrative technique for assigning a shadow address to a mobilestation by the iBS is depicted by flow diagram 800 of FIG. 8. Theprocess starts with processing block 805 when the MS is powered up in awireless region. Next, as evidenced by processing block 810, the MSscans to locate a candidate iBS to serve the MS based upon the value ofthe signal-to-noise ratio (SNR) using a so-called “scanning algorithm”(the scanning algorithm is conventional to a mobile service environment,and it is carried out at the “physical” layer level). Once a candidateiBS is located, processing by block 815 is invoked whereby the MS sendsa “request to associate” with the candidate iBS; the request includesthe MS identifier (as described below) which is unique to the MS. Next,via processing block 820, the candidate iBS assigns a shadow address tothe MS which is compatible with the wireline link layer of the subnet towhich the iBS is connected. The MS may also need to obtain an IP addressif it does not already have one (e.g., when the MS tries to use IPservices for the first time) or if it needs a new IP address (e.g., whenit moves into a new IP subnet). The MS may use any existing methods(e.g., DHCP) to obtain an IP address. In terms of the example used tofill the second row of Table 2, the IP address assigned is 128.44.12.111and the shadow address assigned is F1:F7:04:2D:70:03. Processing block825 is executed so that the candidate iBS stores the shadow address, IPaddress, and the wireless link layer address of the MS in the watch listand the candidate iBS becomes the serving iBS. Finally, as perprocessing block 830, the MS stores the IP address and the shadowaddress to be used during packet processing, as discussed in more detaillater.

In the foregoing the term MS identifier was used, and the following is abrief description of one realization of such an identifier. A wirelessnetwork interface card (NIC) of a MS is assigned a unique address by themanufacturer of the particular NIC—this address is called the “MS MACaddress” or, equivalently, the “MS identifier”, where MAC is the acronymfor Medium Access Control; the MAC Address is utilized at the “link”layer in the wireless network portion of a wireline/wireless network.Each MS identifier usually has 48 bits which can be formatted asfollows: “B1:B2:B3:B4:B5:B6”, where B1, B2, . . . is each one byte.Also, since each byte can be treated as containing two 4-bit nibbles,the MS identifier is such that each nibble can be expressed inhexadecimal. Thus, a typical MS identifier might be:“18:00:20:E8:42:F6”, and it is unique to a particular MS, so it can beused as a universal identifier. In the foregoing description, thesymbolism “xxxxxx” was used to denote the MS identifier, which must becompatible with all iBSs that the MS will roam to in the wirelessnetwork.

Next, the process by which the MS interacts with a new base station toconvey shadow address information pertaining to the MS is considered inoverview fashion in FIG. 9. With reference to FIG. 9, there is shownflow diagram 900 for this process; the flow diagram emphasizes thosedifferences over conventional processing brought about by the use of ashadow address. The process starts with processing block 905 whereby theMS is presumed to be powered up and being served by a base station(referred to as the “old base station” below), as covered by FIG. 8. TheMS continuously monitors, via decision block 910, the incoming signalstrength of the old base station to determine if the SNR falls below aprescribed threshold using the “scanning algorithm”. If the SNR does notfall below a threshold (say 50% of the original SNR ratio), the MScontinues to monitor the SNR. If the SNR falls below the threshold, thenan operational mode of the MS is turned on so that the MS maycommunicate with a base station(s). Then via processing 915, the MSscans, using the physical layer, to locate a new base station with ahigher SNR. Next, decision block 920 is invoked to determine whether ornot a new base station has been located. Whenever a new base station hasbeen located, the MS sends a request (including its shadow address andIP address) to associate with the new base station as evidenced byprocessing block 925. The new base station can either accept or rejectthe request to associate. If rejected, the MS continues to scan for ahigher SNR. If accepted, the new base station updates its watch listwith the shadow address and IP address information. The new base stationsends an acknowledgement of receipt of the information, and the MSawaits an acknowledgement from the new base station so as to turn offthe monitor mode (decision block 930 ). The new base station informs theold base station of the association, via processing block 935; detailsof processing by block 935 from the perspective of the base stations arecovered in FIG. 11. Once soft handoff is complete, then the new basestation will replace the old base station as the serving base station.During soft handoff, the packets being received from the multiple basestations can be used advantageously to determine the true contents ofthe packet from its replicated versions.

Now with reference to FIG. 10, there is shown flow diagram 1000 from theviewpoint a serving iBS upon power-up of a MS within the wireless regionserved by the iBS. Processing starts with block 1005. Next, decisionblock 1010 is entered to determine whether or not this is the firstpower-up of the MS. If not, then there is no further processing for thisMS. If this is the initial power-up, then block 1015 is entered toassign a shadow address to the MS. Then the newly assigned shadowaddress is inserted into the watch list of the iBS, as per processingblock 1020. Finally, the shadow address is transmitted to the MS viaprocessing block 1025. Processing ends with block 1030.

Now with reference to FIG. 11, there is shown flow diagram 1100representative of a candidate base station that is to serve thepowered-up MS as it moves from a serving base station into the overlapof the wireless region of the serving base station and the candidatebase station. Processing starts with block 1105. Next, decision block1110 is entered to determine if there is an approaching MS (known viathe scanning algorithm discussed above). Processing reverts to block1110 if there is no approaching MS. For an approaching MS, viaprocessing block 1115, the shadow address is received from the MS. Thendecision block 1120 is invoked to determine if there is a conflict witha shadow address already in the watch list of the candidate basestation. If there is no conflict, then the original shadow address ishandled by processing block 1125 and is inserted into the watch list ofthe candidate base station. If there is a conflict, processing block1135 is invoked to effect a negotiation between the serving base stationand the candidate base station to determine a suitable replacementshadow address acceptable by both the serving base station and thecandidate base station. Once this negotiation is complete, a new shadowaddress is assigned via processing block 1140, and entered into thewatch list of the candidate base station (as well as replacing the onein the watch list of the serving base station as a result of thenegotiation); also, the candidate base station becomes a new iBS.Processing ends with block 1145.

Referring now to FIG. 12, there is shown flow diagram 1200representative of the process for deleting a shadow address from thewatch list of a base station once the MS moves outside the range of thatbase station. Processing starts with block 1205. Next, decision block1210 is entered to determine is the MS is leaving the coverage range ofthe base station. If not, there is no further processing. If the MS isleaving the coverage range, then processing block 1215 is entered tosignal that a handoff is to be carried out so that the new base station,differentiated from the old serving base station, will now serve the MS.Once handoff is complete, then processing block 1220 is entered todelete the shadow address from the old base station. Processing endswith block 1225.

The technique whereby the new iBS obtains shadow address informationfrom the old iBS, rather than from the MS directly, is now described.The teachings of FIGS. 8–12 can be readily applied to this case. It ispresumed that the MS is already homing on the old iBS and is now roamingto the overlap wireless region also served by the new iBS. The MS scansto locate the new iBS. Once the new iBS is located, the MS sends arequest to associate with the new iBS; the request includes the MSidentifier. The new iBS sends a broadcast message to all other basestations on the subnet to determine which base station has the MSidentifier in its watch list. The old iBS has the MS identifier in itswatch list, so it sends a response to the new iBS with both the IPaddress and the shadow address contained in the old iBS's watch list.Conflicts can be resolved by interchanging messages between the old andnew iBSs, which results in a unique shadow address for the MS now storedin the watch lists of both the new and old iBSs.

2. Soft Handoff Across Subnets

2.1 Packet Distribution Across Subnets

With reference to FIG. 13, there is shown system 1300 that depicts thescenario for soft handoff across Subnets. In particular, iBS A (1311) isconnected to wireline Subnet A (1321), whereas iBS B (1313) is connectedto wireline Subnet B (1325). In turn, Subnet A is coupled to router 1322and Subnet B is coupled to router 1326 (it is possible, without loss ofgenerality, that routers 1322 and 1326 may coalesce into a singlerouter). Both routers 1322 and 1326 are then coupled to IP core network1331. Subnet A, Subnet B, and the IP core network may have any arbitrarynetwork topology. In the arrangement illustrated in FIG. 13, an IPpacket sent via iBS A and iBS B to MS 104 during soft handoff, asdepicted by Packet A (1341) and Packet B (1342), respectively,originates from host 1305 coupled to IP core network 1331. Accordingly,when the mobile station moves across IP Subnets, multiple copies of thesame data are to be sent via multiple base stations to the mobilestation (that is, Packet A and Packet B must be identical).

The manner of achieving the required packet duplication is now discussedfor the following heuristic case: a so-called “nearest router” isresponsible for IP packet duplication and distribution, namely withreference to FIG. 13, router 1322, since this router is “nearest” to theMS 104 and Subnet A brought about by MS 104 initially homing on iBS A.

2.2 Packet Duplication

Once the soft handoff across IP Subnets A and B starts, that is, as MS104 migrates from cell 1301 to cell 1302 in handoff region 1317, iBS A,iBS B, and router 1322 exchange information about MS 104. Theinformation is in a form summarized by packet duplication informationwhich, for the case of the nearest router 1322, that is, the routerinitially handling MS 104, is shown in Table 4.

TABLE 4 MS Link Layer MS's IP Forwarding IP MS's Shadow Address inAddress in Addresses in Address in Wireless Network Subnet A otherSubnets Subnet A xxxxxx IP_(A) IP_(B), . . . MAC₁₀₄ . . . IP₁ IP₂, . . .NIL . . . . . . . . . . . .

The Forwarding IP Address for a MS can be either the IP address used bythe MS directly to receive IP packets in the new subnet or the IPaddress of an agent (e.g., a Mobile IP Foreign Agent or an iBS) in thenew subnet that is responsible for intercepting the IP packets destinedto the MS and then forwarding the packets to the MS.

The process of filling in, for example, row 1 of Table 4 is as follows.First, the procedure for assigning a shadow address to MS 104 in SubnetA has been discussed with reference to FIG. 8. Then the MS sendsinformation in its watch list to nearest router 1322 so this router canbe compiling the duplication table, namely, columns 1, 2, and 4 can befilled in. Referring now to flow diagram 1400 in FIG. 14, the techniquefor associating IP_(B) with MS 104 as served by iBS B and then informingrouter 1322 of the assignment of IP_(B) for entry into column three ofTable 4 is depicted.

The process starts with processing block 1405 whereby the MS is presumedto be powered up and being served by a base station (referred to as the“old base station” below), as covered by FIG. 8. The MS continuouslymonitors, via decision block 1410, the incoming signal strength of theold base station to determine if the SNR falls below a prescribedthreshold using the “scanning algorithm”. If the SNR does not fall belowa threshold (say 50% of the original SNR ratio), the MS continues tomonitor the SNR. If the SNR falls below the threshold, then anoperational mode of the MS is turned on so that the MS may communicatewith a base station(s). Then via processing 1415, the MS scans, usingthe physical layer, to locate a new base station with a higher SNR.Next, decision block 1420 is invoked to determine whether or not a newbase station has been located. Whenever a new base station has beenlocated, the MS sends a request (including its MS identifier and itsIP_(A) address) to associate with the new base station as evidenced byprocessing block 1425. The new base station can either accept or rejectthe request to associate. If rejected, the MS continues to scan for ahigher SNR. If accepted, the new base station (iBS B) sends anacknowledgement that it will associate with the MS. The MS awaits anacknowledgement from iBS B so that the MS may turn off its monitor mode(decision block 1430 ). The MS and iBS B exchange a sequence of messagesusing, for example, the Dynamic Host Configuration Protocol, to assignthe new IP address IP_(B) to the MS for use in Subnet B, as evidence byprocessing block 1435. Then the MS sends a message as part of its normalinterchange with iBS A to inform iBS A of the new IP_(B) address, as perprocessing block 1440. Finally, the iBS A sends a message containing thenew address IP_(B) to the nearest router ( 1322 ), as per processingblock 1445, for completing the remaining entry in the Packet DuplicationTable 4, namely, column three. Processing is ended by block 1450.

Once soft handoff is complete, then the new base station will replacethe old base station as the serving base station. During soft handoff,the packets being received from the multiple base stations can be usedadvantageously to determine the true contents of the packet from itsreplicated versions.

It is possible that more than one “new” candidate base station may belocated during the process of “scanning” for a new base station or basestations. Each new base station independently follows a process aselaborated upon in the foregoing for the interaction between iBS A andiBS B. The interaction of only iBS A and iBS B has been discussed forthe sake of specificity but without loss of generality.

Now, for any incoming IP layer packet arriving from host 1305, router1322 routes, via iBS A, the original Packet A destined for IP addressIP_(A) of MS 104 using its standard routing procedure. In addition,router 1322 duplicates the packet and distributes the duplicate to MS104 as Packet B via iBS B. In general, the procedure is that when anearest router receives a packet destined to the IP addresses in firstcolumn of Table 4, it duplicates the packet and sends the duplicates tothe IP address(es) in the third column of Table 4. Therefore multiplestreams will be sent to all base stations involved in soft handoff.

In some situations, a nearest router may need to respond to the ARPREQUEST to receive the IP packets destined for mobile 104. Therefore theshadow address of MS 104 is maintained in second column of Table 4.Also, if an iBS connects to multiple nearest routers, only one of thenearest routers is chosen to store the shadow address of MS 104. Otherswill simply put NIL in the field, as exemplified by the second row ofTable 4. Similarly, signaling is performed in iBSs A and B and thenearest router 1322 when soft handoff is completed, so the entry of themobile station in a Packet Duplication table will be deleted. Suchsignaling can be done along with the signaling used by the mobilestation to normally perform soft handoff.

EXAMPLE 1

By way of specificity to summarize the procedure step-by-step, considerthe arrangement of FIG. 13 wherein Packet A is sent from correspondenthost 1305 attached to IP core network 1331, that is, the packet has IPdestination address IP_(A) and is sent from “outside” Subnet A. SincePacket A with destination address IP_(A) is sent outside Subnet A, it iseventually routed to router 1322. Once router 1322 receives Packet A, itchecks its Duplication Table. Packet A therefore will be duplicated withIP destination address of IP_(B) as depicted in the third column ofTable 4, and this duplicated packet is be routed to MS 104 via iBS B.Besides, Packet A is routed to MS 104 via iBS A as a standard packet.

If there are multiple nearest routers an iBS connects to, only one ofthem receives the packet sent from host 1305. Therefore only one nearestrouter will duplicate and distribute the packet.

As outlined above, each time a mobile station moves to a new Subnet, itmust acquire a private or public IP address from a server (e.g. DHCPserver or Foreign Agent) for that specific Subnet. This is part of thenormal registration and configuration. However, which IP addresscorrespondent host 1305 should use to reach the mobile station dependson how IP-layer mobility is supported. If, for example, basic MobileIPv4 is employed, host 1305 always uses the home address of the mobilestation. The Forwarding IP Address, IP_(B), for the MS in the new cellwould be the new care-of address the MS obtains for receiving packets inthe new subnet. The normal Mobile IPv4 Home Agent process directs apacket to the MS's care-of address currently registered with the HomeAgent. In this example, the MS can delay the Mobile IPv4 address bindingoperation for its new care-of address IP_(B) until soft handoff iscompleted so that the Mobile IPv4 Home Agent can continue to directpackets destined to the MS to the old base station during the handoff.Router 1322 will then duplicate the packet and forward a copy to the newbase station as described above. Upon completion of soft handoff, the MSwill perform Mobile IP address binding operation for its care-of addressto be used in the new cell so that later packets will be directed tothis new care-of address. To reduce packet loss during the switch overfrom old IP address to the new IP address, removal of the DuplicationTable in Router 1322 may be delayed for a pre-determined or random timeafter the completion of soft handoff so that packets already sent to theold cell can continued to be forwarded by Router 1322 to the new celleven after the MS loses its radio connection with the IBS A.

EXAMPLE 2

For this example, suppose for the moment that host 1305 is connected toSubnet A, that is, Packet A is sent from “inside” Subnet A, so that bothrouter 1322 and iBS A respond to an ARP REQUEST with the mobilestation's shadow address. Therefore, Packet A eventually arrives at bothiBS A and router 1322. The one arriving at router 1322 is forwarded tothe IP address(es) in third column of the Table 4 stored in router 1322.Since this packet arriving at router 1322 is due to the entry in theDuplication Table rather than the normal routing table, router 1322 doesnot perform normal routing so the packet will not be sent to iBS Aagain. In this example, only one of the nearest routers maintains theshadow address in its Duplication Table, so that only one of themduplicates and distributes a packet.

2.3 Distribution of Same Data in Multiple Streams

When the nearest router sends out the duplicated IP packets, thesepackets have different IP addresses of the mobile station so they can berouted to different base stations. To effect soft handoff, packets mustbe exactly the same including any field in the header so combining offields can be done in signal level by the mobile station. Packetsduplicated and distributed from the nearest routers however aredifferent in their destination IP addresses.

However, all the other fields other than the IP destination address aresame when the nearest router duplicated the packets. As described inSection 1, the base stations perform signaling and maintain a Watch Listfor mobile stations involved in the soft handoff process. Base stationstherefore know which mobile stations are currently in the process ofsoft handoff across Subnets. The new base station then changes the IPdestination address of the packet for the mobile station in the WatchLists of the base station to the mobile station's old IP address (IP_A).But instead of broadcasting these IP packets, the base stations willfurther encapsulate the IP packets to link layer frames with the linklayer address in the Watch List as the destination address. These linklayer frames will be sent from base stations to the mobile over the airinterface without broadcasting; upon receipt by the mobile station, thelink layer information is stripped from the frame, leaving only thepacket with the generic broadcast address which is identical for allpackets. The involved mobile station therefore will receive exactly samedata from multiple base stations.

2.4 Packet Selection in Reverse Link

The nearest router described above for packet distribution could be thepoint for packet selection as well, that is, the process of selectingone of the packets arriving from the MS via a corresponding plurality ofbase stations as the propagated pakcet.

For soft handoff in circuit cellular networks, the Nearest Routerreceives RLP (Radio Link Protocol) frames from multiple base stations.In addition to the payload, each RLP frame also comprises of SIR (SignalInterference Ratio), Frame Quality Indicator (FQI), Symbol Error Rate(SER), and so forth. Based on this information, one frame is singled outas the “best” frame for further distribution in the network. To preservesuch layer-2 information, the iBSs encapsulate layer-2 frames to IPpackets, then send them to the Nearest Router, which then decapsulatesthe IP packets, selects one layer-2 frame, and assembles final IPpackets. The restored packets are then routed to the correspondent host(1305) by the Nearest Router. This approach allows the iBSs to performsoft handoff in reverse link in layer-2 as that in today's cellularnetworks. The only added function in iBSs is to encapsulate the RLPframe to IP packets. The Nearest Router, however, will need to performdecapsulation, selection, and IP assembly.

An alternative approach is to generate an IP packet when the iBSreceives a RLP frame. The iBS generates an IP packet with the payload ofthe RLP frame and the decision criteria. Once the Nearest Routerreceives the RLP frame, it can select a packet based on IP payload, thenassemble the original IP packet sent by MS and route it to thecorrespondent host.

Packet selection in the reverse direction has been described withrespect to soft handoff across subnets. It is readily contemplated thatan analogous description applies to packet selection in the reversedirection for soft handoff within a subnet.

3. Data Content Synchronization

One potential way to achieve data content synchronization at the MS isto have all iBSs transmit copies of the same packet to the mobilestation at precisely the same time. However, scheduling the precisetiming for simultaneous transmissions of IP packets on different IPdevices (in this case, iBSs) is very difficult to implement in a real IPnetwork.

This Section describes a new IP-layer procedure, referred to as the“Fluid Synchronization” method, performed by the iBSs to ensure that thedata arriving at the MS at the same time from multiple iBSs are copiesof the same data. The method is an IP-layer procedure and is thereforeindependent of the link layer protocols used in the radio system. Theprocedure is performed by iBSs rather than by the MS, which avoids anymodification to the MS.

Rather than trying to schedule the precise timing for simultaneous IPpacket transmissions on multiple iBSs, the methodology ensures that thestreams of layer-2 data blocks sent by multiple iBSs to the MS are“matchable streams”. Matchable streams are streams of layer-2 blocks (ormore precisely, the physical layer data resulting from these blocks)that can be correctly matched and combined by the MS using today's radiotechnologies (e.g., a RAKE receiver as discussed in the reference by V.K Garg, entitled “IS-95 CDMA and cdma 2000:Cellular/PCS SystemsImplementation”, pp. 60–62, published by Prentice-Hall, 2000).

Suppose that the streams of WP packets sent by different iBSs to the MShave either no gaps (i.e., no missing IP packets) or identical gaps.Then, the layer-2 data block streams from the iB Ss to the MS will bematchable if, for any k, the k^(th) layer-2 data block sent by both iBSsto the MS contains the same amount of payload (i.e., have the samelength). The matching layer-2 data blocks (i.e., data blocks that arecopies of the same data) from different iBSs do not have to arrive atthe mobile at precisely the same time. The mobile's radio system cansynchronize these data blocks using today's radio channelsynchronization techniques, as long as the delay jitters are notexcessively large, which usually is one time slot length of a frame.

It is also important to note that generating matchable streams oflayer-2 data blocks does not require each iBS to send copies of the sameIP packet to the layer-2 protocol at precisely the same time fordelivery to the MS. Furthermore, matchable streams of layer-2 datablocks can be generated by performing only IP-layer processing on theiBSs alone.

FIG. 15 illustrates how matchable streams of layer-2 data blocks can begenerated when the IP packets are sent by different iBSs to the layer-2protocol on their radio interfaces at different times for delivery tothe MS. With reference to FIG. 15, there is shown a stream of numberedpackets, designated “1” (1511), “2”, “3”,“4”, “5”, “6”, and so forth,arriving at, for example, iBS A of FIG. 13. Similarly, the same streamof packets arrives at iBS B, wherein packet 1521 is the first packet inthe stream. Note that the packets arrive at the respective iBSs atdifferent times (time line 1531-1 is used to reference packets for iBSA, whereas replicated time line 1531-2 is used for packets arriving atiBS B). IP packets are sent to the link layer for delivery over-the-airto the MS at the times shown by the downward arrows on the respectivetime lines; for example, packet 1511 is transmitted at the time shown byarrow 1512, and packet 1521 is transmitted at the time shown by arrow1522. The over-the-air link layer receives the packets as data blocksand fills the data blocks into link layer frames. The over-the-air linklayer frames corresponding to iBS A are shown by the stream of framesstarting with 1513, 1514, and so forth. Similarly, the over-the-air linklayer frames corresponding to iBS B are shown by the stream of framesstarting with 1523, 1524, and so forth. Frames 1513 and 1523 are blank,and they are shown primarily to demonstrate that there is a random delay(1532) between the two streams of frames. As long as the random delay iswithin the synchronization capability of the receiver technique, fluidsynchronization can be effected. The “sideways” arrows, such as arrow1515, depict when the iBS A IP packets sent to the link layer fordelivery over-the-air have been fully processed and are encapsulatedinto frames. Thus, frame 1514 encapsulates the data blocks derived frompackets “1” and “2” from iBS A. Similarly, the next frame encapsulatesdata from packets “2” and “3”. Because the data block associated withpacket “5” undergoes a significant delay in delivery, the data block forpacket “5” is not ready for encapsulation until the fifth frame.Moreover, this data block is too long for a single frame, so it is usedto partially fill the next succeeding frame, along with data from packet“6”. Finally, another frame is needed to complete delivery of packet “6”because of its length.

Similarly, the “sideways” arrows, such as arrow 1525, depict when theiBS B IP packets sent to the link layer for delivery over-the-air havebeen fully processed and are encapsulated into frames. Thus, frame 1524encapsulates the data blocks derived from packets “1” and “2” from iBSB. Similarly, the next frame encapsulates data from packets “2” and “3”.Because the data block associated with packet “5” undergoes asignificant delay in delivery, the data block for packet “5” is notready for encapsulation until the fifth frame. Moreover, this data blockis too long for a single frame, so it is used to partially fill the nextsucceeding frame, along with data from packet “6”. Finally, anotherframe is needed to complete delivery of packet “6” because of itslength.

Based on the observations described above, a Basic SynchronizationProcedure (BSP) is as follows (the Radio Link Protocol (RLP) is used asan exemplary radio layer-2 protocol in the following discussions).Starting from the delivery of the same IP packet to the mobile, each iBSwill

-   -   1) Use RLP frames of identical length    -   2) Deliver only fully filled RLP frames to the MS unless        -   a) a timer T_(p) expires, or        -   b) instructed by the upper layer (i.e., the IP layer) to            send the current data.

In real networks, several events may cause a loss of data contentsynchronization when the above method is used. For example, gaps mayrandomly occur in the IP packet streams sent by different iBSs to themobile. Also, when timer T_(p) times out or when the IP layers ondifferent iBSs instruct their layer 2 to send the current availabledata, the resulting layer-2 data blocks from different iBSs may notcontain an identical amount of payload.

To correct for these events, a data content re-synchronization procedureis described that can quickly bring the multiple layer-2 data blockstreams from different iBSs back to synchronization when loss of datacontent synchronization occurs. The foundational principle is that theiBS which detects (or suspects) a loss of data content synchronizationwill negotiate with the other iBSs to restart the BSP procedure from anew IP packet.

This following describes the data content re-synchronization procedureusing packet gaps as an exemplary cause of loss of data contentsynchronization. To help iBSs detect gaps in IP packet streams, thesource or the entity responsible for packet distribution can number thepackets (e.g., using the 16-bit identification field or an optionalfield in the IP header) and increment the packet stream number by oneeach time an IP packet is sent.

The depiction of FIG. 16 is a pictorial representation of the results tobe determined by the re-synchronization procedure. In particular, FIG.16 illustrates two streams of frames sent to the MS from iBS A and iBSB, namely, the stream from iBS A encapsulating packets 1611, . . . ,1612, and the stream from iBS B encapsulating packets 1621, . . . ,1622, . . . , 1623. The frames from iBS B are delayed relative to theframes from iBS A, as already pointed out in FIG. 15. In the depictionof FIG. 16, the frame containing packet “3” from iBS A has been “lost”in the over-the-air delivery from iBS A to the MS. Data contentresynchronization is regained at the RLP frames 1612 and 1623,respectively, for iBS A and iBS B, based upon the above data contentsynchronization procedure now discussed in steps (a)–(f) below.

Suppose that iBS A detects a gap between packet k and packet m (m>k) inthe stream of IP packets destined to the MS. That is, iBS A has receivedpackets k and m but has not received any packet between packets k and m.iBS A will initiate the following data content re-synchronizationprocedure, discussed with reference to flow diagram 1700 of FIG. 17 (asa shorthand, a number x in a layer-2 frame indicates that the layer-2frame contains data from the IP packet with stream number x):

-   -   (a) 1705: iBS A requests iBS B to re-start the BSP procedure        from a packet q (q≧m).    -   (b) 1710: iBS A immediately sends to the MS all the packets it        has received before packet q without enforcing the BSP rules and        halts the delivery of packet q and the packets arrived after        packet q.    -   (c) 1715: determine if iBS B can (or has a high level of        confidence that it can) re-start data synchronization as        requested by iBS A (e.g., when iBS B has received packet q and        has not yet sent it to the MS, or has not yet received packet        q).    -   (d) if so, iBS B will positively acknowledge iBS A's request        (1720). Then, iBS B will immediately send to the MS all the        packets it received before packet q without enforcing the BSP        rules (1725). iBS B will then restart the BSP procedure from        packet q (1730). In other words, layer-2 transmission of packet        q will start from the beginning of a new layer-2 frame after the        packets before q have been delivered. Further, starting from        packet q, layer-2 transmission will follow the BSP rules.    -   (d) 1735: upon receiving positive acknowledge from iBS B, IBS A        will restart the BSP procedure from packet q.    -   (e) if iBS B cannot re-start re-synchronization as requested by        iBS A (e.g., iBS B may have already sent packet q to the MS),        iBS B will select a new packet r after packet q (r>q) and        requests iBS A to start re-synchronization at packet r (1740).    -   (f) both iBS A and iBS B start BSP procedure commencing with        packet r (1745).

The re-synchronization procedure described above can be used to re-gaindata content synchronization when loss of data content synchronizationis caused by other events besides gaps in IP packet streams. If, forexample, T_(p) on iBS B expires before a RLP frame is fully filled, iBSB will send the partially filled frame to the MS. However, this may leadto loss of data content synchronization. To re-gain data contentsynchronization, iBS B can request iBS A to re-start the BSP procedurefrom a new packet. To reduce the impact of loss of data contentsynchronization caused by unexpected events, the iBSs currently involvedin soft handoff may periodically re-start the data contentre-synchronization procedure.

4. Smooth Handoff

This Section discusses how to leverage the Shadow Addresses maintainedin each base station to achieve “smooth handoff”. Smooth handoff meansthat the mobile station still can transmit and receive packets while itis performing handoff. Ideally there will be no delay and packet loss insmooth handoff. Again, a macro-diversity system is assumed, that is, thesystem is such that that a mobile station is capable of transmitting andreceiving data from multiple base stations at the same time. Althoughsmooth handoff in a macro-diversity system is feasible in currentcircuit-based centralized cellular systems, the focus of this Section ison packet-based IP networks in which there is no central controller. Inaddition, the subject matter in accordance with the present inventionhas the following unique features:

-   -   (a) the same algorithms and table can be used for both link- and        network-layer handoffs. The base station does not need to        distinguish the type of handoff, and does not need to run two        handoff algorithms. This makes the IP-based base station        efficient and also reduces the cost.    -   (b) the same algorithms and table can be used for mobile        stations served by multiple base stations either on same or        different IP subnet.    -   (c) no signaling at or above the IP layer is required.    -   (d) a mobile station does not have to use an additional IP        address.    -   (e) no modification to the mobile station is required. Base        stations maintain the necessary table, cache the shadow        addresses, and perform extra techniques for handoff.    -   (f) the smooth handoff technique scales well for large networks.        4.1 Smooth Handoff within a Subnet

Base stations perform the algorithms in Sections 1.1, 1.3 and 1.5 toassign and insert a shadow address in the Watch Lists for the mobilestation. Base stations then respond to an ARP REQUEST when there is anIP packet destined to the mobile station. Since the base stationsinvolving in the smooth handoff have the same shadow address for themobile station, the mobile station will receive the IP packets from atleast one base station. Therefore smooth handoff can be achieved. Theentry of the mobile station in the old base station will be deleted oncethe handoff is done. The mobile station then will receive packets onlyfrom one base station.

4.2 Smooth Handoff Across Subnets

For handoff across different subnets, base stations again perform thesame algorithms in Sections 1.1, 1.3 and 1.5. The shadow addresses usedby the same mobile station in cells belonging to different IP subnetsmay be the same or different. Depending on specific IP-layer mobilitymanagement methods, IP packets may be sent to a single or multiple basestations. In particular, if soft handoff in the IP layer is deployed, IPpackets will be sent to multiple base stations. Smooth handoff can beachieved using the same method described in the Section 4.1. If softhandoff is not deployed in the IP layer, the IP packets will be destinedto only one base station (i.e., either the new or the old base station).Either the new or the old base station will be able to correctly respondto the ARP REQUEST from any other network device (e.g., an IP router oranother iBS) that wants to send IP packets to it. This is because eitherone of the base stations will already have the mobile station's shadowaddress in its Watch List. Therefore, the IP packets can reach themobile station from either one of the base stations. Base stations donot need to do any extra signaling with other base stations in eitherthe link- or the network-layer for carrying out handoff. Furthermore,only one handoff is needed when mobile station moves across IP subnets.Thus, smooth handoff is achieved easily in both link- andnetwork-layers.

Also, by way of reiteration, each time when the mobile station moves toa new subnet, it must acquire a private or public IP address from aserver (e.g. DHCP server or Foreign Agent) for that specific subnet.This is part of the registration and configuration. Which IP address theCH should use to reach the mobile station depends on how location updateis performed. If basic Mobile IPv4 is employed, for example, CH alwaysuses the home address of the mobile station. The normal Home Agentprocess directs a packet to old (or home) IP address before handoff,that is, registration of new IP address with the Home Agent is delayeduntil soft handoff is completed.

In some cases, the mobile station may perform smooth handoff withmultiple base stations. Not all of these cells may be on the same IPsubnet. The algorithms still apply in this case and smooth handoff canbe achieved as well.

Although the present invention have been shown and described in detailherein, those skilled in the art can readily devise many other variedembodiments that still incorporate these teachings. Thus, the previousdescription merely illustrates the principles of the invention. It willthus be appreciated that those with ordinary skill in the art will beable to devise various arrangements which, although not explicitlydescribed or shown herein, embody principles of the invention and areincluded within its spirit and scope. Furthermore, all examples andconditional language recited herein are principally intended expresslyto be only for pedagogical purposes to aid the reader in understandingthe principles of the invention and the concepts contributed by theinventor to furthering the art, and are to be construed as being withoutlimitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, that is, any elements developed that perform the function,regardless of structure.

In addition, it will be appreciated by those with ordinary skill in theart that the block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.

1. A method for carrying out soft handoff of a mobile station from afirst base station served by a first wireline subnet to a second basestation served by a second wireline subnet, wherein each subnet has alink layer different than the link layer of the wireless network servingthe mobile station, the mobile station has initially a first IP addresscompatible with the first wireline subnet, and each base station has awireline link layer address, and wherein the first base station isserved by a nearest router, the method comprising assigning by the firstbase station a shadow address for the mobile station, the shadow addresscorresponding to the mobile station, having a format compatible with thelink layer of the first wireline subnet, and being distinct from saidbase station layer addresses and said mobile station first IP address.storing the first IP address and the shadow address in the nearestrouter, assigning a second IP address in the mobile station compatiblewith the second wireless subnet, communicating the second IP addressfrom the mobile station to the nearest router via the first basestation, and transmitting a frame containing a packet as conveyed by asending device from the nearest router over the first wireline subnet tothe first base station as determined by the shadow address and over thesecond wireline subnet to the second base station based upon the secondIP address.
 2. The method as recited in claim 1 further includingcommunicating the shadow address from the first base station to thesending device in response to an address resolution request by thesending device.
 3. The method as recited in claim 1 further comprising,after the transmitting concurrently sending the packet from both thefirst base station and the second base station to the mobile stationusing a link layer frame compatible with the link layer of the wirelessnetwork.
 4. The method as recited in claim 1 wherein the storingincludes storing the first IP address and the shadow address for themobile station as entries in a watch list in the first base station. 5.The method as recited in claim 1 further comprising, prior tocommunicating, sending the shadow address from the first base station tothe mobile station and storing the shadow address in the mobile station.6. A method for carrying out soft handoff of a mobile station from afirst base station served by a first wireless subnet to a second basestation served by a second wireline subnet, wherein each subnet has alink layer different than the link layer of the wireless network servingthe mobile station, and the mobile station has initially a first IPaddress compatible with the first wireless subnet, and each base stationhas a wireline link layer address, the method comprising assigning ashadow address to the mobile station, the shadow address correspondingto the mobile station, having a format compatible with the link layer ofthe first wireline subnet, and being distinct from said base stationlayer addresses and said mobile station first IP address, identifying anearest router serving the first base station, storing the first IPaddress and the shadow address of the mobile station in the nearestrouter, assigning a second IP address to the mobile station compatiblewith the second wireless subnet, communicating the second IP addressfrom the mobile station to the nearest router via the first basestation, and transmitting a frame containing a packet as conveyed by asending device from the nearest router over the first wireline subnet tothe first base station as determined by the shadow address from thefirst base station to the sending device in response to an addressresolution request by the sending device.
 7. The method as recited inclaim 6 further including communicating the shadow address from thefirst base station to the sending device in response to an addressresolution request by the sending device.
 8. The method as recited inclaim 6 further comprising, after the transmitting, concurrently sendingthe packet from both the first base station and the second base stationto the mobile station using a link layer frame compatible with the linklayer of the wireless network.
 9. A method for carrying out IP layersoft handoff of a mobile station from a first base station served by afirst wireline subnet to a second base station served by a secondwireline subnet, wherein each subnet has a link layer different than thelink layer of the wireless network serving the mobile station, themobile station has initially a first IP address compatible with thefirst wireless subnet, and each base station has a wireline link layeraddress, the method comprising identifying a nearest router serving thefirst base station, assigning a shadow address to the mobile station bythe first base station, the shadow address having the same format as thelink layer address of the first wireline subnet and being distinct fromsaid base station layer addresses and said first IP address of saidmobile station, storing the shadow address and the first IP address inthe nearest router, associating the mobile station with the second basestation including the assignment of a second IP address to the mobilestation, communicating the second IP address from the mobile station tothe nearest router via the first base station for storage, transmittinga frame containing a packet as conveyed by a sending device from thenearest router over the first wireline subnet to the first base stationas determined the shadow address and over the second wireline subnet tothe second base station based on the second IP address, and propagatingthe packet from the first base station to the mobile station using theIP layer of the wireless network, and concurrently propagating thepacket from the second base station using the IP layer of the wirelessnetwork.
 10. The method as recited in claim 9 wherein the sending devicesends an address resolution request including the first IP layer addressof the mobile station and further including looking up the shadowaddress corresponding to the first IP layer address as stored in thenearest router and sending the shadow address in response to the addressresolution request.
 11. The method as recited in claim 9 wherein thestep of propagating the packet from the first base station includesremoving the packet from the wireline frame, passing the packet to theIP layer of the first base station, encapsulating the packet as a linklayer wireless frame, and propagating the link layer wireless frame overa radio channel coupling the first base station with the mobile station.12. The method as recited in claim 9 wherein the step of propagating thepacket from the second base station includes removing the packet fromthe wireline frame, passing the packet to the IP layer of the secondbase station, encapsulating the packet as a link layer wireless frameand propagating the link layer wireless frame over a radio channelcoupling the second base station with the mobile station.
 13. The methodas recited in claim 9 wherein the step of propagating the packet fromthe first base station includes removing the packet from the wirelineframe, passing the packet to the IP layer of the first base station,encapsulating the packet as a link layer wireless frame, and propagatingthe link layer wireless frame over a radio channel coupling the firstbase station with the mobile station, and wherein the step ofpropagating the packet from the second base station includes removingthe packet from the wireline frame, passing the packet to the IP layerof the second base station, encapsulating the packet as a link layerwireless frame, and propagating the link layer wireless frame over aradio channel coupling the second base station with the mobile station.14. A system for carrying out soft handoff of a mobile station from afirst base station served by a first wireline subnet to a second basestation served by a second wireline subnet, wherein each subnet has alink layer different from the link layer of the wireless network servingthe mobile station, wherein the mobile station has initially a first IPaddress compatible with the first wireline subnet, wherein the firstbase station is served by a nearest router, and wherein each basestation has a link layer wireline address, the system comprising astorage device for storing the first IP address and a shadow address inthe nearest router, the shadow address corresponding to the mobilestation, having a format compatible with the link layer of the firstwireline subnet being distinct from said base station link layeraddresses and said mobile station first IP address, and being assignedby the first base station, a processor for assigning a second IP addressto the mobile station compatible with the second wireline subnet, asending device for communicating the second IP address to the nearestrouter via the first base station, and a transmitter for transmitting aframe containing the packet as conveyed by the sending device from thenearest router over the first wireline subnet to the first base stationas determined by the shadow address and over the second wireline subnetto the second base station based upon the second IP address.